With development of micromachining technologies, the importance of actuators has been increasing in various fields. An example of the field in which microactuators are used is, for example, an optical switch that is used in optical communication or the like to switch the optical path. Examples of such an optical switch include, for example, optical switches disclosed in Japanese Patent Application Laid-Open No. 2001-42233 and the pamphlet of International Publication WO03/060592.
A microactuator generally has a fixed portion and a movable portion that is adapted to be movable relative to the fixed portion, and the movable portion can be moved and retained at a predetermined position by a driving force applied thereto.
In a microactuator that utilizes as the driving force an electrostatic force, a fixed electrode and a movable electrode are provided in the fixed portion and the movable portion respectively. In such a microactuator, since an electrostatic force is generated between the electrodes by applying a voltage between the electrodes, advantages such as simplification of the structure can be achieved. For this reason, in many conventional microactuators, an electrostatic force is utilized as the driving force.
In a microactuator for moving a micro mirror used in an optical switch disclosed in Japanese Patent Application Laid-Open No. 2001-42233, an electrostatic force is used to move a movable portion to a predetermined position against a spring force acting on the movable portion and to retain it at that position. A microactuator used in an optical switch disclosed in the pamphlet of International Publication WO03/060592 is adapted to be capable of utilizing, as a driving force, a Lorentz force in addition to an electrostatic force. For example, a movable portion is moved to a predetermined position by a Lorentz force against a spring force acting on the movable portion, and the movable portion is retained at that position by an electrostatic force.
In conventional microactuators that utilize an electrostatic force including the microactuators disclosed in Japanese Patent Application Laid-Open No. 2001-42233 and the pamphlet of the International Publication WO03/060592, the fixed electrode and the movable electrode are arranged in such a way that they overlap each other for the most part as seen from the direction of movement of the movable portion.
In conventional microactuators utilizing an electrostatic force, since the fixed electrode and the movable electrode are arranged in such a way that they overlap each other for the most part as seen from the direction of movement of the movable portion, when an electrostatic force is generated, the state in which the movable portion is in contact with the fixed portion (which state will be referred to as “pull-in state” in this specification) occurs, and it has not been possible to retain the movable portion stably at a position before it comes in contact with the fixed portion in the state in which an electrostatic force is generated.
In conventional microactuators utilizing an electrostatic force, troubles have occurred, or their uses have been limited due to occurrence of the above mentioned pull-in state.
For example, since the movable portion is pressed against the fixed portion in the above mentioned pull-in state, the movable portion may stick to the fixed portion to become inoperable, or even if it does not become inoperable, operation delay may occur due to time taken in detaching the movable portion from the fixed portion.
Furthermore, for example, due to occurrence of the above mentioned pull-in state, it is not possible to control the position of the movable portion in such an analogue manner as to change the stop position of the movable portion to a desired position according to the magnitude of the voltage applied between the fixed electrode and the movable electrode, conventional microactuators that utilize an electrostatic force have had only limited applications such as optical switches which can be satisfactorily position-controlled in a digital manner.